Mice born with combined lipase deficiency (cld/cld) have very low levels of lipoprotein lipase and hepatic lipase activities, develop extreme hypertriglyceridemia, and die within 3 days. The recessive mutation (cld) causing this deficiency is located on chromosome 17, whereas structural genes for the lipases are located on chromosomes 9 and 11, respectively. Primary cultures of brown adipocytes derived from tissue of newborn mice were used to study the effect of the cld mutation on lipoprotein lipase. Cells cultured from brown adipose tissue of cld/cld mice replicated, became confluent, and differentiated into brown adipocytes at the same rates as cells of unaffected mice. Unaffected adipocytes synthesized active (dimeric), secretable lipoprotein lipase (Mr=58000) which contained two complex oligosaccharide chains and could be released from cells by heparin. Cld/cld adipocytes also synthesized fully glycosylated lipoprotein lipase (Mr=56000), some of which was dimerized, but the lipase contained only high mannose type oligosaccharides, and was inactive and not secretable. Immunolocalization studies showed that the lipase in cld/cld adipocytes was retained in endoplasmic reticulum. Unaffected brown adipocytes treated with swainsonine, an inhibitor of mannosidase II in Golgi, synthesized a high mannose type lipoprotein lipase which was both active and secreted, indicating that trimming/processing of high mannose oligosaccharide chains in Golgi may not be necessary for activity or secretion of lipoprotein lipase. Whether the cld mutation affects primarily processing of oligosaccharides in endoplasmic reticulum, transport of lipase from the reticulum, or some other process, is to be resolved.